1. Field of the Invention
The invention relates to plastic optical fiber.
2. Discussion of the Related Art
Glass optical fiber has become a significant transmission medium in recent years, particularly for long distance transmission applications. Such optical fiber has not found significant usage, however, in smaller scale applications, such as distribution of fiber to the desk in local area networks. In particular, glass optical fiber has not been as cost effective as, for example, copper wire, and also requires extremely precise fiber connections, e.g., end face polishing, alignment, and index-matching material. There has been interest, therefore, in pursuing plastic optical fiber (POF), which offers many of the benefits of glass optical fiber, but is expected to offer more cost effective systems. POF also offers some unique characteristics, including a larger core and desirable dispersion properties, which are expected to make connection and splicing easier.
However, connectivity approaches, e.g., termination and connection techniques, used for glass optical fiber are not necessarily desirable for plastic optical fiber. Yet, such basic issues with respect to plastic optical fiber systems must be resolved in order for POF to achieve commercial acceptance. For example, current POF connections tend to exhibit undesirably high losses, e.g., 2 to 3 dB. Thus, improved techniques for terminating POF are desired, advantageously techniques that result in low-loss connections.
The invention relates to POF processes and systems and involves improved termination techniques that make dry, non-polished connection more acceptable. The techniques provide good physical characteristics, i.e., smoothness, at the termination end face, thereby providing lower loss connections than conventionally obtained. For example, losses less than 1 dB have been obtained without polishing or index-matching material, e.g., for CYTOP(copyright) fiber having a polymethylmethacrylate reinforcement (CYTOP(copyright) is poly(perfluoro-butenyl vinyl ether), and is available commercially from Asahi Glass Co., Japan).
According to one embodiment, POF is cut while the fiber is under axial compression, and the usable piece (or pieces) is typically removed prior to pulling back the blade or knife. The resulting termination exhibits a smooth surface that promotes low loss in a dry, non-polished connection. According to another embodiment, the POF is cleaved. Specifically, the fiber is notched and then pulled at a relatively high strain rate to induce fracture. The rate is such that the strain remains in the elastic region up to and during fracture, i.e., the fiber exhibits brittle, as opposed to ductile, behavior during the strain and fracture. The brittle behavior is necessary for a smooth termination surface that similarly promotes low loss in a dry, non-polished connection.
These techniques of the invention overcome problems discovered to be inherent in prior, conventional cutting and cleaving techniques. For example, cleaving with slow strain rates led to poor surfaces not suitable for connection without further processing. And, significantly, cutting in tension or even in non-compression, was found to lead to branching cracks within the fiber, i.e., cracks propagating into the fiber from the end face. Branching cracks in particular are believed to contribute to the relatively high losses currently found in POF connections. The invention, by providing improved termination and thus easier, lower-loss connections, enhances the ability of POF to be incorporated into optical communication systems.